Genetic variation among populations of Pythium irregulare in southern Australia

Abstract

Isolates of Pythium irregulare were sampled from seven cereal crops throughout South Australia to determine the extent of genetic diversity within this pathogen and the scale of genetic differentiation among populations. Data derived from 29 individual restriction fragment length polymorphism (RFLP) loci differentiated 54 DNA fingerprints among the 92 isolates analysed. Some isolates had two alleles at several RFLP loci and were scored as heterozygous. One such isolate was selfed in vitro and segregation ratios in the progeny were not significantly different from those expected for allelic variation in a diploid. These data provided evidence that outcrossing occurs within P. irregulare and may contribute to the high level of genetic variation within the species (D(T) = 0.502). Allelic frequencies were significantly different among all seven populations and G(ST) values showed significant genetic differentiation between populations. The average genetic identity among populations was low and hierarchical cluster analysis provided no clear evidence that populations formed geographically related groups. These analyses indicate low levels of interpopulation gene flow within P. irregulare and imply that population differentiation results from genetic drift.